1. Field of the Invention
The present invention relates to a honeycomb structured body and an exhaust gas purifying device.
2. Discussion of the Background
In recent years, particulates, contained in exhaust gases discharged from internal combustion engines of vehicles such as buses, trucks, construction machines and the like, have raised serious problems as these particulates are harmful to the environment and the human body. There have been proposed various honeycomb structural bodies used as ceramics filters which allow exhaust gases to pass through porous ceramics to collect particulates in the exhaust gases, so that the exhaust gases can be purified.
Heretofore, there is disclosed a ceramic filter wherein through holes an end of each of them on a side from which exhaust gases flow out is sealed (hereinafter referred optionally to as “through hole on flow-in side”) are made to be through holes of a large volume (hereinafter referred optionally to as “large-capacity through hole”), while through holes an end of each of them on a side to which exhaust gases flow in is sealed (hereinafter referred optionally to as “through hole on flow-out side”) are made to be through holes of a small volume (hereinafter referred optionally to as “small-capacity through hole”), whereby an aperture ratio on the flow-in side of the exhaust gases is made relatively larger than that of the flow-out side of the exhaust gases.
FIG. 18 is a cross-sectional view showing schematically a cross-section perpendicular to the length direction of a conventional exhaust gas filter.
This exhaust-gas filter 60 has a cross-sectional structure in which squares, each smaller than each of square shapes constituting the checker board pattern, are placed on intersections of the checker board pattern, and this structure is constituted by small-capacity through holes 61b corresponding to the smaller squares and large-capacity through holes 61a located on the periphery thereof, with the partition wall 62a, 62b being formed between these through holes (see U.S. Pat. No. 4,417,908 and JP-A 58-196820).
FIGS. 19A to 19D are cross-sectional views each showing schematically a cross-section perpendicular to the length direction of another conventional exhaust filter.
These exhaust gas filters 300 to 330 are composed of large-capacity through holes 301a, 311a, 321a, and 331a having various shapes, respectively, and small-capacity through holes 301b, 311b, 321b, and 331b having various shapes, respectively. Portions of the partition wall 302, 312, 322, and 332 are formed between these through holes, respectively.
Any of the portions of the partition wall 302, 312, 322, and 332 is the one for separating each of the large-capacity through holes 301a, 311a, 321a, and 331a and each of the small-capacity through holes 301b, 311b, 321b, 331b, respectively. In this respect, it may be said that no partition wall for separating the large-capacity through holes 301a, 311a, 321a, and 331a from one another exists (see U.S. Pat. No. 4,364,761, JP-A 56-124417, JP-A 62-96717, U.S. Pat. No. 4,276,071 and JP-A 56-124418).
Furthermore, a filter wherein a cell pitch of large-capacity through holes is made to be almost 1.0 to 2.5 mm is disclosed as another prior art (see J UM-A 56-187890 microfilm (J UM-A 58-92409 (see page 4, FIG. 6))).
Besides, a filter wherein a volume ratio in large-capacity through holes is 60 to 70%, a volume ratio in small-capacity through holes is 20 to 30%, and a cell pitch of the large-capacity through holes is almost 2.5 to 5.0 mm is also disclosed (see JP-A 5-68828 (Japanese Patent No. 3130587 (page 1))).
FIG. 20 is a cross-sectional view showing schematically a cross-section perpendicular to the length direction (hereinafter optionally referred simply to as “cross-section”) of each of the above-mentioned filters 200. In the filter 200, small-capacity through holes 202 each having a triangular cross-section are disposed around each large-capacity through hole 201 having a hexagonal cross-section.
Moreover, a filter having 40 to 120% of a percentage of a ratio of a gross area in cross-sections of small-capacity through holes with respect to a gross area in cross-sections of large-capacity through holes is also disclosed (see JP-A 2001-334114 (see page 5, FIG. 2) and International Publication No. WO02/100514).
FIG. 21 is across-sectional view that schematically shows a cross-section perpendicular to the length direction of such a honeycomb structured body, and in a filter 210, small-capacity through holes 212, each having an laterally elongated hexagonal shape in its cross section, are placed on the periphery of a large-capacity through hole 211 having a right hexagonal shape in its cross section. Moreover, in the periphery of the circumference thereof, the large-capacity through holes 211 having a right hexagonal shape and large-capacity through holes 213 having a trapezoidal shape are placed also in parallel with each other.
Moreover, a filter wherein the number of through holes on its flow-in side is made larger than that of its flow-out side, whereby an aperture ratio on the flow-in side of exhaust gases is made to be relatively larger than that of the flow-out side of the exhaust gases is also disclosed (For example, see FIG. 3 of JP-A 2001-334114).
On one hand, a technique for adjusting a thickness of a wall, and physical properties of a filter are also disclosed (see U.S. Pat. Nos. 4,416,676 and 4,420,316). A filter containing through holes having square and rectangular cross-sections is also disclosed (see JP-A 58-150015).
A filter containing through holes having two types of shapes, among others, octagonal and quadrangular shapes is also disclosed (see French Patent No. 2789327 and International Publication No. WO02/10562). Another filter containing through holes having two types of shapes, i.e. relatively large square and small square shapes is disclosed (see International Publication No. WO03/20407).
In addition, a honeycomb filter having 1.1 to 15 times higher aperture ratio on a flow-in side of exhaust gases than that of a flow-out side of the exhaust gases is also disclosed (see International Publication No. WO03/80218).
The contents of U.S. Pat. No. 4,417,908, JP-A 58-196820, U.S. Pat. No. 4,364,761, JP-A 56-124417, JP-A 62-96717, U.S. Pat. No. 4,276,071, JP-A 56-124418, JUM-A56-187890microfilm, Japanese Patent No. 3130587, JP-A 2001-334114, International Publication No. WO02/100514, U.S. Pat. Nos. 4,416,676, 4,420,316, JP-A 58-150015, French Patent No. 2789327, International Publication No. WO02/10562, and International Publication No. WO03/80218 are incorporated herein by reference in their entirety.